In device for continuously treating metal strip, in particular a metal strip made of aluminum (or an aluminum alloy) or nonferrous metal (or a nonferrous-metal alloy) typically has at least one heat-treatment device through which the metal strip is passed without contact, and comprising a strip-centering device that adjusts the position of the metal strip within and transverse to the strip-travel plane with or without feedback. The heat-treatment device has at least one heating zone on the upstream inlet end and one cooling zone on the downstream outlet end. The metal strip preferably has a thickness of 0.1 mm to 6 mm.
The heat-treatment device is preferably a noncontact tunnel furnace having a heating zone and a cooling zone. The heating zone usually consists of a plurality of heating subzones (heating and/or holding zones) and the cooling zone usually consists of a plurality of cooling subzones. In such a heat-treatment device, the metal strip is heated to a certain (target) temperature, optionally held at this temperature for a certain period of time and then cooled again. The strip passes through the furnace without contact by suspending the strip between fluid jets from nozzles supplied with appropriately pressurized fluid. The cooling in the cooling zones may be done by air or water or a combination of air and water. Such noncontact tunnel furnaces having a heating zone at one end and a cooling zone at the other end are known (see DE 198 04 184 [U.S. Pat. No. 6,413,470] for example).
Such an apparatus of the above-described type for continuously treating metal strip comprising a heat-treatment device and/or a noncontact tunnel furnace may be, for example, an annealing line and/or a continuous annealing line in which the metal strip is heat treated for metallurgical purposes, for example, to achieve certain strength and deformation properties. Alternatively, however, the apparatus may be a strip-coating system and/or a strip-coating line in which the metal strip is not heat treated for the purpose of annealing but instead to dry a coating on the strip, so that the furnace is then a continuous is dryer.
The metal strip is preferably an aluminum strip or a nonferrous metal strip with a thickness of 0.1 mm to 6 mm.
In annealing lines, for example, the metal strip is heated to temperatures approaching the melting point, so it is usually necessary to set a relatively low tension in the heat-treatment device to prevent the strip from rupturing. The strip tension is dissipated in a tension roller set at the upstream intake end, for example, and then after cooling, it is built up again at the downstream outlet end at another tension roller set. In the heat-treatment device (noncontact tunnel furnace), the specific strip tension amounts to 0.5 to 1 MPa, for example. The strip may “run off center” in particular at low tension in the furnace, for example, due to strip defects, if any, so it is necessary to position the strip in a suitable manner with the help of a strip adjuster, preferably positioning the strip centrally. Consequently, the positioning of the strip is performed transverse to the strip-travel direction and within the strip-travel plane. Such a strip-centering device usually has at least one control roller as well as suitable position sensors (e.g. strip edge detectors). With the systems known in practice, the strip-centering device is downstream of the heat-treatment device, i.e. downstream of the cooling zone. The control roller in practice is usually embodied as a so-called PI strip center regulation, i.e. using a proportional P-component and an integral I-component. The I-component is in the furnace, thereby preventing the strip from running too much off center in the furnace. The control roller usually sits on a movable base frame, which causes the roller to rotate about an imaginary center of rotation and/or about an imaginary axis of rotation situated within the furnace section, where it is perpendicular to the strip-travel plane. Detection of displacement of the roller out of the central axis of the furnace section is the proportional amount while the measure of the skewed position of the roller is the integral amount of the strip center regulation. With the roller positioned at a skewed angle, the strip travels back in the direction of the center of the strip due to the so-called winding effect. Such systems that are known in practice have proven to be fundamentally suitable.
A system of the type defined in the introduction is known from DE 103 37 502, for example. A deflecting roller that serves to control the center of the strip is provided downstream of the furnace having heating zones and cooling zones.
In practice there is a need for more efficient and more productive continuous annealing lines due to the rapidly growing demand for automotive body sheets made of aluminum. To achieve higher production capacities, the strip passes through the treatment section at a higher rate. However, since only a limited heat can be imparted to the strip in each furnace zone, it follows from this that the heat-treatment device would have to be designed with a greater length for a higher production capacity. Since the strip runs off center in the furnace section more easily due to the low strip tension, there is the risk with long furnace lengths that the known strip-centering devices will no longer be sufficient to keep the strip travel stable in the furnace, so there is the risk of the strip running off center laterally and/or running up against the furnace structure. This could then lead to unwanted damage to the strip or to a rupture of the strip, so systems with an increased production capacity cannot be readily implemented in this way. This is where the present invention begins.